The instant invention generally relates to a method of determining the quality of fish and more specifically, to a method to determine the quantitative distribution and rate of migration of trimethylamine in fish.
This invention was conceived and developed largely for tuna and tuna-like species because of the special problems encountered with the preservation of both the dark and light meat present therein. Therefore, it will be explained largely with respect to tuna and tuna-like species, although it can be used for other types of fish, i.e., haddock, cod, pollack, cusk and hake, in the broader aspects of the invention.
Tuna and tuna-like fish, most often, are caught in the ocean many miles from port. The fish are then placed into a brine solution in holds in the fishing vessel and subsequently frozen when the holds are filled. The freezing may occur within a matter of a few hours or days, depending upon the availability of fish. The tuna may deteriorate if not frozen quickly or thoroughly or if exposed to air for long periods of time. The quality of fish is determined by organoleptic evaluation after they have thawed and on the production line prior to canning.
Organoleptic methods of analyzing tuna fish have been accepted as the standard in the industry for determining the quality of tuna fish prior to precooking in the canning process. These methods include the evaluation of individual tuna fish for rancid or putrid odors, discoloration, soft mushy skin, bent or deformed carcasses and other indicators of bacterial contamination which can be recognized by the human senses. In the present practice of this conventional process, a skilled worker doesn't have many problems in determining high quality and low quality fish. However, it is virtually impossible for the worker to discover a fish of marginal quality, or one that is about to become low quality. Additionally, many fish are often discarded because of a bent, deformed or mangled carcass even though the quality of the meat may be very high.
It's been recognized in the industry that the presence of amines in fish is related to the deterioration thereof. For example, in an article entitled, "Assessment of Green Tuna: Determining Trimethylamine Oxide and its Distribution in Tuna Muscles", by M. Yamagata et al, appearing in Journal of Food Science, Vol. 34, p. 156, (1969), it is stated that the off-color "greening" condition of pre-cooked tuna is very closely related to the trimethylamine oxide content of the raw meat.
In another article entitled, "Production of Dimethylamine In Muscle of Several Species of Gadoid Fish during Frozen Storage, Especially In Relation To Presence Of Dark Muscle" by C. H. Castell et al, appearing in the Journal of The Fisheries Research Board of Canada, Vol. 28, No. 1, January 1971, it is stated that dimethylamine was produced in the muscle of five gadoid species during storage at -5.degree. C. When the dark lateral muscle was removed prior to freezing, the production of the dimethylene was either greatly inhibited or reduced. A further article by T. Tokunaga entitled, "Trimethylamine Oxide And Its Decomposition In The Dark Muscle Of Fish," in the Bulletin of the Japanese Society of Scientific Fisheries, Vol. 36, No. 5, p. 510 (1970) expresses a similar view. In that publication, it was reported that dark and light muscles of different species of fish were kept separately but under similar conditions in order to examine changes in content of trimethylamine, dimethylamine and formaldehyde during storage for 25 days at 0.degree. C. No change occurred, but a fairly rapid increase in amines was observed in the dark muscle of all the samples. At -6.degree. C., trimethylamine content was somewhat less than that produced at 0.degree. C., but dimethylamine slowly increased. The formation of the amines in the dark muscle was considered to be caused by enzymatic action and was thought to be used as in index to fish freshness prior to spoilage by bacterial growth.
However, there is a need for a method to chemically analyze the quality of fish prior to processing and particularly to identify the rate and extent of migration of trimethylamine from dark to light meat in fish to detect low quality fish that may pass organoleptic evaluation.